Motor vehicles are increasingly equipped with automatic clutches which can be designed, like in the manner of construction of a single or multiple-disc dry clutch equipped with a pressure spring, as a passively engagable or actively disengageable friction clutches, or, as in the manner of construction a wet-running multiple-disc clutch, as actively engagable or passively disengageable friction clutches. With the use of an automatic clutch, the driver is freed, when starting or shifting, from the performance of a coordinated engaging and disengaging of the clutch and can concentrate on other control tasks. In addition to an increase of driving comfort, a reduction of fuel use and damaging emissions from the vehicle in question can be obtained by the use of an automatic clutch, especially in conjunction an automatic standard transmission.
In order to guarantee comfortable and low-wear operation of an automatic friction clutch, an exact as possible knowledge of the association between a regulating parameter of an associated clutch actuator acquired by a sensor or a transmission element placed between the clutch actuator and an activation element of the friction clutch, like a regulating path, a regulating pressure or an electric regulating current, and the required torque conveyed from the friction clutch in the respective operating position. This combination, at least in the form of a characteristic torque curve as the function of a regulating parameter, preferably a regulating path, is deposited in a data storage facility of an associated control unit from whence the clutch regulator in question can be controlled.
The value of that regulating parameter of the friction clutch (designated among other things as the touch point, the point of engagement, kiss point, the grip point or the measuring point) forms an essential reference point for the torque curve of a friction clutch at which with an activated clutch regulator in the engaged sense the motor-side and transmission-side friction elements of the clutch align and come in frictional contact so that the friction clutch just begins to transmit a minimal torque and where with an activated clutch regulator in the disengaging direction, the motor-side and transmission-side friction elements of the clutch are just separated so that the frictionally-engaged conveyance of a torque by the friction clutch ends precisely.
While the torque gradient, i.e. the inclination of the torque curve, practically depends only on the friction value between the friction elements of the clutch, and thus is almost constant within the control range of the friction clutch, the touch point and thus the control range of the torque curve can displace as a dependency of relevant operating parameters, like the operating temperature of the clutch and the wear condition of the friction coverings. Furthermore, there can be deviations with respect to the touch points within a series production of friction clutches of the same design because of manufacturing tolerances.
For comfortable and low-wear operation of a friction clutch and the start-up and shifting procedures controlled by it, as exact a knowledge as possible of the respectively current touch point is required to which the torque curve of the friction clutch can be adapted. That applies in particular for the two friction clutches of a dual-clutch transmission in which at least one clutch is used as a start-up clutch and both clutches can alternately be engaged and disengaged overlapping in time. By means of a possibly frequent and exact adaptation of the torque curve of both friction clutches of a dual-clutch transmission, a constant driving and shifting quality of the vehicle in question is striven for in all operating conditions and for its entire useful life.
Accordingly, several procedures for a clutch characteristic curve adaptation of a, automatic dual-clutch transmission are proposed which are based on the fact that with a running motor and disengaged gears in a load-free sub-gearbox, a current value of the touch point of the associated friction clutch with which the torque characteristic curve is to thereafter be adapted is determined in conjunction with a partial engagement of the friction clutch in question and the acquisition of relevant operating parameters.
Thus in a known procedure, for example, for a clutch characteristic curve adaptation of an automatic dual-clutch transmission according to WO 2003/074 895 A2, it is proposed that the current touch point of the friction clutch of a sub-gearbox (when the vehicle is stopped, a main brake or parking brake is activated, the motor is running and a gear is engaged) can be determined by the acquisition of the current value of the regulating parameter of the clutch regulator by means of the engagement of the clutch until the attainment of an increase of the motor torque of the drive motor and the torque characteristic curve in question is thereby adapted.
The known procedure indeed includes a zero point adjustment of the clutch regulator; nevertheless, the determination or adaptation of the touch point is only possible when a vehicle is stopped and idling control of the drive motor is activated, and is thus relatively seldom possible. In addition, the known procedure is critical for safety, since too great an engagement of the friction clutch, i.e., too high a clutch torque, can result in an unintentional rolling of the vehicle, whereas too small an engagement of the friction clutch, and likewise a too small clutch torque, can result in a relatively too small increase of the motor torque that can not be distinguished from other impairments of the idling revolutions of the drive motor.
In another known method for clutch characteristic curve adaptation of an automatic dual-clutch transmission, it is provided in WO 2001/014 761 A1, that with disengaged gears and a sufficiently high rotational speed difference at the friction clutch of the currently load-free sub-gearbox, the friction clutch is slowly engaged, whereupon the input shaft of the transmission in question is accelerated to the motor speed of the drive motor after exceeding the touch point. The variations in time of the input shaft rotation speed and the regulating parameter of the associated clutch regulator are acquired and stored. The touch point of the applicable friction clutch is then determined from the association of the rotation speed gradients of the input shaft and the regulating parameter of the clutch regulator in a manner not further described. In the event of an initially too low rotational speed difference, it should be increased by starting the synchronization of the gear clutch of a gear of the applicable sub-gearbox. The known method can be used both with a stopped vehicle and when driving, but features a relatively high degree of inexactness in the determination of the touch point because of multiple influence parameters.
It is envisioned in a partially similar method for clutch characteristic curve adaptation of an automatic dual-clutch transmission according to EP 1 067 008 B1 that with disengaged gears and a sufficiently high rotational speed differential on the friction clutch of the momentarily load-free sub-gearbox, the friction clutch is first partially engaged with a defined clutch regulating force, whereupon the associated input shaft of the transmission is accelerated to the rotational speed of the drive motor. Thereafter synchronization of a gear clutch of the same sub-gearbox is started with a defined regulating force, by means of which a synchronous torque exceeding the clutch torque of the friction clutch is produced, and the rotational speed of the input shaft separates itself or differs from the motor rotational speed. After a sufficiently high rotational speed difference is reached on the gear clutch, the regulating force is again slackened to the synchronization of the gear clutch so that the rotational speed of the input shaft again approximates the motor rotational speed under the effect of the clutch torque of the friction clutch.
A rotational speed gradient is determined from the rotational speed progression of the input shaft of the transmission acquired by sensor and stored in memory by means of which the current clutch torque of the friction clutch is determined under consideration of the inertia moment of the input shaft and a drag or braking moment acting on the input shaft and from that the actual touch point of the friction clutch is derived.
This known method is accordingly relatively time-consuming and disadvantageously requires a high storage and calculation capacity. The set clutch torque of the friction clutch must also be comparatively low, since otherwise it can not be overcome by the temporarily set synchronous torque of the synchronization of the gear clutch or there is the danger of an unintentional complete engagement of the gear clutch. Such a determination of the touch point of the friction clutches is accordingly relatively inexact.
The previously named method for characteristic curve adaptation of an automatic dual-clutch transmission is changed in WO 2004/076 225 A1 in that after partial engagement of the friction clutch with a defined clutch regulating force, the synchronization of a gear clutch of the same sub-gearbox is activated with a continuously increasing regulating force until the synchronous torque of the gear clutch exceeds the clutch torque of the friction clutch. In this operating state, the rotational speed of the applicable input wave is separated from the motor rotational speed, which is recognized by signals of the associated rotational speed sensors. The current torque of the friction clutch is determined by the association of the current regulating parameter of the gear regulator or the synchronous torque produced thereby to the current regulating parameter of the clutch regulator with which the current touch point of the friction clutch can be determined or adapted. In contrast to the named method, the sequence of this known method is simplified and accelerated to be sure; the disadvantages caused by the low regulating force level, however, remain unchanged.